1. Field of the Invention
The present invention relates to a switching circuit for balancing battery cells, and more particularly to a switching circuit for interrupting balancing current.
2. Description of the Prior Art
There are various methods of balancing cells of lithium ion battery. The most used one among the various methods is to apply an electric current to a battery cell having a relatively high voltage in a discharge direction, so as to balance battery cells.
Typically, a method of applying electric current to a resistor, etc. is merely used to balance battery cells. However, in this method, electric power is greatly wasted. Therefore, this method is used for a system consuming a small amount of electric power. Further, there is another method using a DC-DC converter. Generally, this method has a high efficiency and a low heating value.
However, since it is impossible for a battery to have an efficiency of 100%, there is a problem in that the total voltage of the battery cell may be lowered below the original minimum voltage due to the balancing of the battery cells when the number of battery cells having a low voltage increases. In addition, the cells may be efficiently balanced by combining charging current and discharging current.
General balancing control circuits have an independent balance electric source (including resistor, etc.) disposed in each battery cell so as to allow electric current for balancing to flow, and alternately provide the balancing current to various cells in a manner of time division.
FIG. 1 is the block diagram illustrating the structure of a conventional balancing control circuit. The conventional balancing control circuit will be described with reference to FIG. 1.
Terminal voltages of cells B1, B2, B3 and B4 of a lithium ion battery are selected by a line selector 10, and then provided through a ground shift 20 to a central processing unit (CPU) 30. An analog to digital converter (A/D converter) embedded in a CPU 30 A/D converts the analog terminal voltages into digital data, so that they become readable as digital data.
The CPU 30 compares voltage data of the cells B1, B2, B3 and B4 of the lithium ion battery with one another, so as to obtain differences between the values of the voltage data. If the differences are greater than a prescribed value, it is determined that balances between the cells are different.
Further, the CPU 30 provides a balance control signal to a balance current controller 40 in order to balance the cells, so as to allow the balance current controller 40 to apply electric current to the cells to be balanced.
The balance current is applied from a balance electric source (for example D-D converter) to the cells selected by an electric current switch. Switches selecting a terminal of the battery cell are generally arranged as shown in FIG. 2. The terminals B1, B2, B3 and B4 are sequentially connected to Bv+ and Bv− according to the operation of the switches.
Such an arrangement is realized by Metal Oxide Silicon Field Effect Transistors (MOSFETs) having a parasitic diode, as shown in FIG. 3.
At this time, the MOSFETs S2, S3, S6 and S7 need to interrupt the bi-directional flow of electric current, and each MOSFET is disposed between drains (or sources) so as to connect the drains (or sources) to each other, thereby preventing the flow of the current caused by the parasitic diode.
In this case, a dozen MOSFETs are required.
Further, in consideration of voltage applied to MOSFETs between drain and source, the maximum voltage is applied to the MOSFETs S1, S4, S5, and S8. The voltage is three times greater than the voltage of the cell. For example, assuming that the maximum voltage of each cell is about 4.5V, the sum of the voltage of three cells is about 13.5V. Since the MOSFETs having tolerance of internal voltage are generally required, MOSFETs having a voltage level of 20˜30V are used.
In the conventional balance control circuit as described above, since the MOSFET must have higher internal voltage than total voltage of the battery cells, MOSFETs having high internal voltage are required and a great number of MOSFETs are needed in order to interrupt the flow of the electric current through the parasitic diode.